We need to know Model and Serial numbers to get the correct image for your deck. I did a Google image search for "John Deere R72 parts diagram" and got hundreds of different images for decks.
On my deck I do not see the opening for the cotter key to hook the spring to , alsoHere is an explode view. Hope it helps.View attachment 22556
Ok the spring is there to create tension for the deck belt so the blade will spin. it looks as though it connects to part #21. I am not sure of this but from the picture that is where I would start looking. Whole in deck could be plugged with grass or dirt. Just poke around a little with a screwdriver and see if you can find it.